Building a Cold Frame to Jump Start your Spring Garden

Wouldn’t you love to extend your growing season, as you would with a greenhouse, but on a smaller, more affordable scale? If you answered “yes,” consider adding a cold frame to your repertoire of gardening tools!

Simply put, a cold frame is a greenhouse on a very small scale. It is usually a rectangular box less than 2 feet tall, and of a small enough scale to be able to reach every part of the inside from the outer edges without stepping inside of it and endangering the very plants you are trying to protect. There are many different designs and materials suitable for building one, depending on your level of skill and how much you are willing to spend!

In concept, a cold frame is simple. It has a clear "lid" that allows the sunlight to reach your plants and warm the soil. That heat is then reserved overnight by the closed box, keeping the inside of the box 5 to10 degrees warmer than the outside air. You can even insulate your cold frame by surrounding it with bales of straw, or covering it with blankets or burlap bags of leaves overnight.

When designing a cold frame, one common method is to find or buy an old storm window or storm door, which will determine the size of your cold frame. If you want a larger cold frame, use two windows set side by side, separately hinged. The "corners" designed for raised beds work well for a "no skill" way of joining the sides of a square cold frame. They add a bit more to the cost, but are readily available, and are a simple way to construct a sturdy base.

Make the side walls at an angle, descending about an inch per foot of length on your side panels. This helps the rain drain off the top, and increases sun exposure as well. Your finished box will look somewhat like a jeweler's display case, with hinges along the taller back side. Here are two examples, constructed by Dave's Garden members.

You may hover your mouse over any image for a description, or click on it to see it enlarged.

Coldframe by SallyG, using a storm door

Coldframe by Yardqueen1948, using several windows

SallyG designed the frame on the left to provide a 45-degree angle to maximize the amount of sunlight collected. You will need to include a support to prop the lid open, so you can provide ventilation, and keep both hands free to work on plants without dropping the heavy lid on your head! Concussions are never a pleasant experience, and we don't want your first blossoms to be purple blooms on your forehead!

Other options I have seen include stapling heavy plastic to a simple wooden frame, sometimes with an inner support system of chicken wire behind the plastic sheeting.

My mother, Edy Hill (also a Dave's Garden subscriber) is a veteran cold-frame user. Never one to accept the status quo, she decided to "build a better mouse trap" as it were, and put her design skills to work. She built me a fantastic cold frame as a Christmas present (see the top photo), inspiring this article, and eliciting envy and pattern requests from her gardening friends! It looks just like a miniature domed or Quonset style greenhouse. A wooden rectangle forms the base with arches supporting the clear plastic dome.

She built my frame out of 2x4's finished with an exterior water-proofing sealer; hers is painted with a good grade of white exterior paint. She chose untreated wood due to concern about chemicals leaching out of treated lumber, since we plan to grow vegetables and herbs. She also made one for a friend using 25-year-old deck boards from a deck he had replaced. Theoretically, anything harmful that might leach out would already be gone from its years of exposure. Although more expensive, deck boards of synthetic material would be long lasting and wouldn't need painting. This would also eliminate concerns with chemicals leaking from pressure treated lumber.

Here are the materials needed, and the basic steps of construction:

• Two 8-foot lengths of 2x4 or 2x6 lumber. Select untreated lumber if your coldframe is for edibles.• Two 6-foot lengths of 1x4 or 1x6 lumber.• Three 8-foot lengths of 1-inch wide flexible plastic lattice molding strips• 4 yards heavy exterior grade flexible plastic in 54-inch width (Note: Edy purchased plastic designed for making covers for outdoor furniture or picnic table covers, available at Hobby Lobby and at many large fabric stores.) • Heavy-duty thread--some are available that are UV resistant for exterior use.• ½ inch metal grommets (like the ones used for the top of a shower curtain) available at fabric stores and craft departments• 2 ½ inch deck screws for joining corners.• Exterior wood glue• Exterior waterproof sealer or exterior paint if you wish to finish it. (Pressure-treated lumber or deck boards would not need finishing).• Stakes (tent stakes with a hook or notch work well for securing the plastic so it doesn't blow away)

The cost will vary depending on the materials used. Edy spent $25 to $30 on the cold frames she made from new lumber.

Here are the steps to build your cold frame:

1) Cut 26 inches from each 2 x 4 x 8 to form the two end pieces. The remaining 70-inch lengths form the sides.

2) Cut the 1 x 4 the length of the sides (70 inches plus the thickness of the 2 ends equals approx. 73 inches). 3) Evenly space five slots in the 1 x 4s for the plastic lattice arches, beginning one inch from each end, at approximately 14-inch intervals. Using the plastic lattice strip as a guide for the width (ours was 1-inch wide), mark the locations of your cuts. Set the blade on your table saw or circular saw to cut a ¼ inch depth. By making multiple, closely spaced ¼ inch deep cuts across the grain of the board, cut each groove ¼ inch deep and 1-inch wide. Repeat for all five slots on this board. If you prefer to use a router, set the bit for ¼ inch. Double-check the width to make certain the plastic lattice can slide in and out of the notches. Mark the second board to match the first, and repeat steps.

4) Join the 26-inch lengths to the ends of the 70-inch side boards with the decking screws to form a rectangle. (See diagram below.)

5) Using exterior glue and small nails,attach the 1 x 4 to the outside of your rectangular frame, with the grooves toward the frame, forming the slots for the arched lattices supports. Your final rectangular frame will measure 27 3/4 inches wide by 72 inches long by 2 inches tall on the outside edges, and should look something like the pictures below. Hover your mouse over them for a caption, and click to enlarge.

6) Cut the 8-foot long plastic lattice molding strips into half, giving six, 4-foot long pieces. Use five of these for the arches, slipping the ends into the slots on one side and curving them to fit in the slot on the opposite side.

To make the plastic cover:

1) Hold a sheet of poster board or cardboard against the end of your cold frame, and draw a pattern for the ends of the plastic cover. Trace the curve of the arch, extending it down over the outside of the boards to the ground. Smooth the curve and add a 1-inch seam allowance and 2 1/2 inches for a hem along the straight bottom edge. Cut out the cardboard pattern, using a box cutter or razor knife. Lay your pattern on the plastic. Draw around the "half circle" arched pattern for the two ends.

2) Using a square to assure the corners are at a right angle, draw a line with a permanent fine tip marker to straighten the end of the plastic. Measure the length of the cold frame and add 2 inches for the main section of the cover, drawing your lines with a straight edge and a permanent marker. This will give you a rectangle large enough to drape over the hoops, reaching the ground on both long sides, and extending about 1 inch past the last hoop on each end for your seam allowance.

3) For the casing, draw lines for a strip 5 to 6-inches wide across the entire width of the remaining 54 inches of plastic. Cut out the plastic on the lines you have drawn, for the main rectangle, end pieces, and strips. Cut the strips in half, giving you two 27-inch long strips.

4) Sewing the plastic is probably the most difficult step. Since the plastic tends to stick to the sewing machine, a Teflon adhesive strip matching the size and shape of your presser foot (available with a peel-off adhesive back) can be obtained from your sewing machine dealer. These are typically used for sewing leather garments. Tape a piece of heavy paper to the sewing machine with an opening cut just large enough for the feed dogs.

5) Fold up a hem 1 1/4-inches wide along the 6-foot long sides of the plastic, then fold up again to form a double hem. Stitch. Hem the straight lower edge of the curved end panels in the same way.

6) Sew one curved edge of the end panels to the short side of the main panel, leaving the other curved edge free for the opening. The stitching line is marked in bold black lines along the curve in the diagram, below. Click to enlarge. 7) Add a casing to the end panel along the opening, indicated in red in the diagram. To do this, fold the 5 to 6-inch strip in half lengthwise. Starting about 6 inches from ground level, stitch the cut edges to the open portion of the end panel. The end lattice strips go through this casing to hold the plastic on the end panels securely in place.

8) Insert plastic lattice strips into the grooves on both sides of the wood frame. For each end, slide the end lattice strips through the casing on the open edge of the cover before inserting into the frame (see picture, right). Mark the placement for grommets at the ends and by the arched supports. This will allow you to secure the plastic to stakes like a tent so it doesn't take flight during windy weather.

9)To install the grommets, purchase ½-inch metal grommets and a simple grommet tool (available at minimal cost at most fabric stores and the fabric and craft section of large stores). Place the two halves of the grommet on opposite sides of the plastic. Use a hammer and the grommet tool as directed in the instructions included with the grommets, and tap the two halves together.

Although we haven't yet tried it, we are considering using an idea from another website, using curved plastic window well covers for the ends. While more costly, it would eliminate the difficulties involved with sewing the plastic cover.

To access the flats, the plastic can be rolled up on one side. It can also be opened only an inch or so for ventilation on a sunny day, or tied open to varying degrees to allow excess heat to escape when the sun does shine. Getting too warm on sunny days is as much of a concern as getting too cold. Note the section of the end flap that is not sewn closed, left.

The genius of the design is that the frame takes almost no room to store during the off-season: the frame hangs flush against our garage wall on bicycle hooks, only extending 4 inches away from the wall. The trellis strips remove easily with no fasteners and take little room to store, and the plastic cover folds neatly into a small square. It is much lighter and easier for a single person to set up and take down than one based on a heavy glass window and deeper wooden frame. It also allows you to grow much taller plants than a standard cold frame, as it is about 18 inches tall at its highest point!

To read the second article in this series, on how to set up and use a cold frame, click here:

I'd like to extend special thanks to my mother, Edy Hill, for inspiring me to extend my own gardening into the cooler months by building me a cold frame! She is my inspiration, and was one of my chief resources in writing this article!

I also owe thanks to yardqueen1948 and sallyg, for allowing me to include their coldframe pictures, and Jill Nicolaus (Critterologist) for the diagram of how to cut the slots for the lattice strips. I appreciate all the Dave's Garden writers who read and offered suggestions on how to make the directions more understandable!

Happy Gardening!

About Angela Carson

Angie was bitten hard by the gardening bug when she was just a child, and has been doing her best to infect as many people as possible ever since! She particularly has a passion for spring bulbs and home-grown vegetables, which she is teaching the next generation how to preserve. Her two sons have obviously inherited her interest in growing things, and her husband is starting to see the benefits of less lawn to mow, as long as he doesn't have to do the work of digging up new beds for her latest schemes! Follow Angie on Google.